1. Field of the Invention
The present invention generally relates to disk drives, and in particular, to read error recovery in a read operation using a GMR read head.
2. Description of the Related Art
In recent years, disk drives, typified by hard disk drives, have employed a magnetic head having a giant magnetoresistive (GMR) read element (also referred to as a GMR sensor) and a write element separately provided in the same slider in order to promote high-density recording.
The GMR read element operates as a read head to read data recorded on the disk medium. The write element operates as a write head to generate a recording magnetic field in accordance with a write current to magnetically record data on the disk medium.
The GMR read element is generally designed so that two ferromagnetic layers are arranged across a non-magnetic layer. One of the ferromagnetic layers is called a free layer and is magnetized in a fixed direction when no external magnetic fields are present.
The other ferromagnetic layer is called a pinning layer and is magnetized in a fixed direction perpendicular to the magnetizing direction of the free layer when no external magnetic fields are present. In general, the application of external magnetic fields does not vary the magnetizing direction of the pinning layer. To fix the magnetizing direction, an antiferromagnetic layer called a fixed layer is stacked on the pinning layer in direct contact with it. The fixed layer provides a fixing magnetic field to fix the magnetizing direction of the pinning layer on the basis of exchange coupling.
In the GNR read element, the magnetizing direction of the free layer varies depending on the magnetic flux direction of magnetic recording with respect to the surface of the disk medium. Furthermore, resistivity varies depending on the magnetizing angle between the free layer and the pinning layer. In this case, the GMR read element is pre-supplied with a bias current (sense current) and can thus read data magnetically recorded on the disk medium as a variation in magnetizing direction.
When for example, ESD (electrostatic discharge) or disturbance noise (crosstalk or the like) causes temperature of the GMR read element to increase above the blocking temperature of the antiferromagnetic layer, the magnetizing direction of the antiferromagnetic layer is reversed. Thus, the magnetizing direction of the pinning layer, exchange-coupled to the antiferromagnetic layer, is also reversed. Here, the blocking temperature means the temperature at which the exchange anisotropy of the antiferromagnetic layer disappears.
Furthermore, if the disk drive undergoes temperature stress for a long time owing to the physical contact between the GMR read element and the disk medium, heating resulting from a bias current, or the like, then the initial direction of the pinning layer cannot be fixed because of surrounding magnetic fields. Thus, for the GMR read element, a read signal output may be degraded or the waveform of an output signal may vary. Consequently, the GMR read element cannot provide its predetermined performance.
When the performance of the GMR read element is thus degraded by the reversal of the pinning layer, a read error may occur during a read operation of reading user or servo data from the disk medium.
To solve such a problem, a method has been proposed which senses that the magnetizing direction of the pinning layer in the GMR read element has been reversed and which then applies a predetermined current pulse to recover the original magnetizing direction (refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 11-191201 or U.S. Pat. No. 5,650,887).
Another method has been proposed which simultaneously provides a bias current and a write current to recover the magnetizing direction of the pinning layer (refer to, for example, Jpn. Pat. Appln. KOKAI Publication No. 10-49837 or U.S. Pat. No. 5,969,896).
However, with the above prior art methods, when pinning reset is executed to recover the magnetizing direction of the pinning layer, if a current pulse is applied as a reset pulse, then the GMR read element may be physically broken down. Furthermore, the method of simultaneously applying a bias current and a write current may create similar problems even if a bias current is supplied which has a current value excessively larger than a normal one.